I can offer from experience with a 1980 Chris Craft 45' aft cabin with those engines, at 10 miles an hour you will get about one mile to the gallon. Up on plane the number will drop to about .4 miles per gallon.

The Bertram 46 has more dead rise than the Chris so the up on plane number might be slightly lower.
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I'd try the Bertram Owners forum, if there is one. I haven't seen many boats like yours here. I had 8v71n's in this Hatteras before I repowered, but they are much less horsepower. Your 8v71ti's are rated at 450hp.

I traveled around 1200 miles last year with my Bertram, same engine package. All was at 1200 rpm to save fuel. We still struggled to achieve one mile per gallon at 9 knots. I know now that a rpm reduction might be a big help. 1000 rpm will still be in 7-8 knot range. Cannot afford to test at top speed...... great boat just not efficient as a LRC